Folded slot antenna

ABSTRACT

A folded slot antenna comprising a sheet of conductive material folded upon itself and having a bow tie shaped slot arranged symmetrically about the bend of the fold on the surface of the sheet. This antenna structure can be flush mounted, for example, in the leading edge of the horizontal stabilizer or of the dorsal fin of an aircraft.

United States Patent Al'lhlll' R. Whitty Hollis, N.ll.

July 24, 1970 Nov. 23, 1971 Sanders Associates, Inc. Nashua, N.ll.

[72] Inventor [2l Appl. No. [22] Filed [45] Patented [73] Assignee [54] FOLDED SLOT ANTENNA 4 Claims, 5 Drawing Figs.

[52] US. Cl 343/767, 343/708 [51] Int. Cl. ll0lq 1/28, HOlq 13/10, l-lOlq 13/16 [50] Field of Search 343/705,

[56] Reierences Cited UNITED STATES PATENTS 3,475,555 10/1969 Bassen et al. 343/908 x FOREIGN PATENTS 803,723 10/1958 Great Britain 343/708 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Marvin Nussbaum Altomey-Louis Etlinger ABSTRACT: A folded slot antenna comprising a sheet of conductive material folded upon itself and having a bow tie shaped slot arranged symmetrically about the bend of the fold on the surface of the sheet. This antenna structure can be flush mounted, for example, in the leading edge of the horizontal stabilizer or of the dorsal fin of an aircraft.

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RECEIVER 25 INVENTOR. ARTHUR R. WHITTY (Mam A T TORNE Y roman SLOT ANTENNA BACKGROUND OF THE INVENTION desirable since externally mounted antennas increase drag and Y are subject to icing conditions, which in some cases cause the aircraft rudder to lock.

2. Description of Prior Art One of the problems encountered in the design of flushmounted antennas for the aviation, navigation or communication band is the large physical size required to achieve wide band performance. In particular, the antenna aperture or slot dimension must be on the order of )t/2(where his wavelength) at the center frequency of the band of interest. For example, in the navigation band; the slot length for a slotted antenna must be on the order of 50inches. It is known that the efiective aircraft surface area occupied by the slot length can be reduced by various techniques such as ferrite loading, folded slots, dumbbell slots and the like. However, techniques such as the foregoing tend to decrease the radiation efficiency such that the antenna gain is reduced by essentially the same factor as the bandwidth is increased. That is, the overall gain is the product of the directive gain product the radiation efficiency.

BRIEF SUMMARY OF THE INVENTION An object of this invention is to provide a new and improved folded slot antenna.

Another object of this invention is to provide a novel and improved antenna which can be flush mounted on a moving vehicle such as an aircraft.

Yet another object is to provide an electrically small antenna structure.

Still another object is to provide an improved wide band antenna which is electrically small in physical structure but yet achieves the efficiency of a slotted antenna having a slot length of lt/2at the center frequency of the band of interest.

Antenna apparatus embodying the present invention comprises a sheet of conductive material folded upon itself and having a bow tie shaped slot arranged symmetrically about the bend of the fold in the surface of the sheet.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings, like reference characters denote like structural elements, and:

FIG. 1 is a plan view of an unfolded bow tie antenna structure viewed from the electrically conductive ground plane side of the structure; I

FIG. 2 is a cross section of a portion of the antenna structure taken along the lines 2-2 of FIG. 1, which section illustrates the sandwich-type nature of the antenna structure;

FIG. 3 is a phantom or outline view of the ground plane folded back upon itself with the bow tie shaped slot arranged symmetrically about the bend of the fold;

FIG. 4 is a side view of a portion of an aircraft illustrating the fiush mounting of an antenna embodying the invention in the aircraft dorsal fin for an aviation communication band application;

FIG. 5 is a block diagram in part and a perspective view in part illustrating the flush mounting of antenna structures embodying the invention in the horizontal stabilizer of an aircraft for an aviation navigation band application.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIGS. 1-3 there is shown an antenna aparranged symmetrically about the bend 12 of the folded ground plane so as to apportion the ground plane 10 into an outer portion and an inner portion 10b. For wide band performance the slot It has a total length of A/2at the high end of the band of interest. However, due to the bow tie shape, the slot 11 occupies a rather small area of the surface of ground plane 10. Energy feed means, such as coaxial cable 13 is coupled across slot 11 at the narrow extreme of the bow tie near or at the bend area 12.

In FIG. 1 the ground plane 10 is shown in a plan view of an unfolded position so as to more clearly show the slot 1] and its electric field directions. For reference purposes the bend 12 of FIG. 3 is formed by folding the top and the bottom of the ground plane 10 out of the plane of FIG. 1 drawing toward the viewer about the dash line UP. When the antenna structure is excited by a signal from a transmitter/receiver device, not shown, via cable 13, the resulting electric field has a direction which is always across the slot it between its inner and outer edges. When the ground plane 10 is not folded as in FIG. 1 the electric fields in the elongated slot portions R-S and XW tend to cancel one another. However, when the slot is folded as in FIG. 3, the fields in the elongated slot portions -RS and XW are additive such that there is a resultant E vector having a direction along those planes which are parallel to the orientation of the bend 12 (FIG. 3).

The elongated slot portions R-8 and XW are preferably chosen as long as possible (within the A/Zdesign length) such that the electric field induced across these elongated slot portions overcomes the cancelling field due to the slanted slot portions P-Q, T-U, U-V, PY. For example, the electric field at 14 of slot portion TU has the first component El which is perpendicular to the field of portion R-S and a second component E2 which is parallel to and in opposition to the electric field across the portion R-S. Since the magnitude of the E2 component is less than the magnitude of the E field in the elongated R-S slot portion, there will be a resultant electric field. In addition, the field in the S'I' portion of the slot 11 has a field component parallel to and in the same direction as the field of slot portion R-S, as illustrated at point 15. These field components in the S-T portion are also additive to the resulting direction of the E vector so as to tend to overcome the cancelling effects of the field induced in the TU portion. An analysis similar to the foregoing can also be made for the PQR, PYX and the UVW slot portion for those field components which are parallel to the direction of the E vector with the'end result that there will be a resultant E vector having a direction along those planes which are parallel to the orientation of the bend 12 of FIG. 3.

Those field components in slot portions PQR and STU which are perpendicular to the field contributed by elongated slot portion R-S oppose one another. Assuming a uniform electric field across the slot 11, the last-mentioned field components tend to cancel one another due to the geometrical symmetry of the bow tie shaped slot 11. A similar analysis could be pursued to demonstrate that those field components in the slot portions PYX and UVW which are perpendicular to the field contributed by the XW elongated slot portion tend to cancel one another.

Thus, the polarization of the antenna (E-vector direction) is therefore dependent upon the orientation of the bend 12 of the folded ground plane 10. Further, electric fields due to signals from a polarization opposite to a chosen one will be substantially cancelled. For example, the antenna structure may be mounted on an aircraft dorsal fin so as to have a vertical polarization for operation in the communication band as illustrated in FIG. 4. 0n the other hand, the antenna structure could be mounted upon the aircraft horizontal stabilizer as shown in FIG. 5 so as to have a horizontal polarization for operation in the navigation band. In either case, the intended result is that electric fields due to signals of the opposite polarization are substantially cancelled.

Reference is now made to FIG. 2 which is a partial cross section of the antenna apparatus as shown in FIG. 1 along the lines 2-2 illustrating an exemplary structurally sound or rigid structure for supporting the ground plane 10. As shown in FIG. 2, the ground plane may be suitably composed of a metallic sheet of material as for example a sheet of aluminum. The metallic sheet 10 is mounted upon a rigid structure which is shown to comprise sheet substances 16 and 17. According to one mode of manufacture, the sheet substance 16 could be a sheet of flexible plastic. The metallic sheet 10 is first laminated to the flexible plastic sheet 16. The slot 11 would then be etched according to well-known processes. The laminated plastic sheet and metallic ground plane 10 would then be bonded at its free surface to a sheet of rigid material 17. The rigid material 17 may be any suitable electromagnetically transparent material such as fiber glass. Prior to the bonding operation, the sheet 17 is preferably formed into a folded structure the degree of bend of which is chosen to fit the contour of that portion of the aircraft or other vehicle where it is to be flush mounted. The bonding operation it then performed, after which the antenna structure is ready for flush mounting.

With reference to FIG. 4 the antenna apparatus may be flush mounted for vertical polarization (communication band operation) in the dorsal fin 18 of an aircraft of which only a portion is shown. In FIG. 4 the antenna apparatus is designated by the reference character 19 and has the same bow tie slot as described above. The dorsal fin 18 is shown to have two portions 18a and 18b. The portion 18a is mounted forward of the antenna 19 while the portion 18b is mounted aft of the antenna 19. The metallic ground plane and the antenna structure 19 make contact with the metallic skin of the aircraft fuselage.

Referring next to FIG. 5, antenna apparatus embodying the invention is illustrated as adapted for flush mounting in an aircraft horizontal stabilizer for a navigation band application. In this application a pair of substantially identical antenna elements 20a and 20b, each embodying the invention, are adapted for flush mounting in a pair of precut receptacles 21a and 21b, respectively, in a horizontal stabilizer 22. The energy feed cables 23a and 23b of the antenna elements 200 and 20b, respectively, are coupled to a power combiner 24 which combines the received energy. The power combiner, of course, is coupled to a receive device 25.

The precut receptacles 21a and 21b are located on difi'erent sides of the tail structure and therefore the fuselage (neither of which is shown in FIG. 5) of the aircraft. The reason for two elements is that is has been found that one element does not operate efficiently throughout the navigation band due to the shading effects of the fuselage. The ground plane of each of the antenna elements 200 and 20b makes contact with the metallic skin of the aircraft horizontal stabilizer. The rib structure within the stabilizer unit 22 tends to provide cavity backing for the folded slot antenna elements 20a and 20b.

There has been described a novel and improved antenna apparatus in which a sheet of conductive material is folded upon itself and has an endless bow tie shaped slot about the bend of the fold. The polarization of the antenna element is along those planes which are parallel to the orientation of the bend of the folded conductive sheet. Preferably the slot length is )t/2but can be loaded is desired with an attendant loss in antenna efficiency. For even wider band performance, the slot length may be made longer than M2. The width of the slot is preferably uniform so as to maintain symmetry.

What is claimed is:

1. An antenna comprising a sheet of conductive material folded upon itself and having a bow tie shaped endless slot arranged symmetrically about the bend of the fold on the surface of the sheet.

2. The invention according to claim 1 wherein said slot has a uniform width.

3. The invention according to claim 2 wherein said conductive sheet is a sheet of metallic material.

4. The invention according to claim 3 wherein said sheet of metallic material is affigced to a sheet of rigid electromagnetically transparent material which has the same folded contour as the metallic sheet.

l t it 

1. An antenna comprising a sheet of conductive material folded upon itself and having a bow tie shaped endless slot arranged symmetrically about the bend of the fold on the surface of the sheet.
 2. The invention according to claim 1 wherein said slot has a uniform width.
 3. The invention according to claim 2 wherein said conductive sheet is a sheet of metallic material.
 4. The invention according to claim 3 wherein said sheet of metallic material is affixed to a sheet of rigid electromagnetically transparent material which has the same folded contour as the metallic sheet. 